1. Field of the Invention
The present invention relates to an injection molding machine, and in particular to an excessive load detecting device for detecting an excessive load exerted on an injection screw when the screw is moved with resin remaining unmolten in a heating cylinder.
2. Description of Related Art
FIG. 5 schematically shows a melting state of resin in a heating cylinder of an injection molding machine. In FIG. 5, an injection screw 3 is inserted into a heating cylinder 1 having a nozzle 2 at a distal end thereof with an appropriate clearance therebetween. Heaters 5 such as band heaters are arrange around the heating cylinder 1 and also the nozzle 2. Resin pellets 7 are supplied into the heating cylinder 1 from a hopper 6. The resin pellets 7 are molten by heat from the heating cylinder 1 and by frictional heat produced by kneading by rotation of the screw 3, and accumulated in the distal end of the cylinder 1. The accumulated molten resin is injected into a cavity of a mold by moving the screw 3 forward in its axial direction.
In FIG. 5, the molten resin 7a is shown as a part with light daub in the cylinder 1 and the unmolten resin pellets 7b are shown as a part with dark daub in the cylinder 1.
If the molten resin 7a remains in the cylinder 1 when an operation of the injection molding machine is stopped after predetermined injection molding operations are finished, the molten resin 7a is solidified with decrease of temperature of the cylinder 1. In restarting molding operations, an excessive load is exerted on the screw 3 if the screw 3 is moved with the solid resin remaining unmolten at the distal end of the cylinder 1.
Thus, it is necessary to determine whether or not normal driving of the screw 3 for axial and rotational motions thereof is possible when restarting the operation of the injection molding machine. Conventionally, this determination is carried out based on confirmation that the temperature of the heating cylinder 1 reaches a predetermined value. In the state where the solid resin 7a remains in the cylinder 1 to fix the injection screw 3 to the cylinder 1, an excessive stress is produced on the screw 3 to possibly cause a considerable damage on the screw 3 and a screw head thereof by an axial or rotational motion of the screw 3. To avoid such an event, the driving of the screw 3 is started after confirming that the temperature of the cylinder 1 has reached to the predetermined value in the conventional practice. In this way, the load on the screw 3 is presumed indirectly based on the temperature of the cylinder 3.
If the temperature of the cylinder 1 reaches the predetermined value, the solid resin 7a is not molten remaining the solid state or the solid resin 7a is molten to have such a high viscosity as to cause an excessive load on the screw and the screw head. The excessive load causes high pulling stress, pressing stress and torsion stress on the screw and the screw head and may cause fatigue failure of the screw and the screw head.
In view of the above, it has been adopted a method in which the driving of the screw is started when predetermined time elapses after the temperature of the cylinder reached the set temperature so that the temperature in the cylinder reaches a target value. Further, there is proposed in Japanese Pat. No. 2,597,922, a method of determining viscosity of the resin in the cylinder based on comparison between a position deviation between a position command and an actual position when the screw is driven at low speed and low torque in the backward direction for a predetermined time period, and a predetermined value. If the resin remains umolten in the cylinder when the screw is driven at low speed and low torque, the motor can not rotates at the commanded speed to make the deviation between the position command the actual position. Thus, the load exerted on the screw is determined based on the comparison between the position deviation and the predetermined value and if the position deviation does not exceed the predetermined value, it is determined that the resin has been molten to have an adequate fluidity.
In the method of starting the driving of the screw when the predetermined time elapses after the temperature of the cylinder reached the set temperature, the load on the screw is presumed indirectly and there is no assurance that no excessive load is exerted on the screw. Thus, the actual temperature and fluidity of the resin are not certain because these factors are influenced by ambient temperature and humidity, etc. Further, there may be a case where an accurate temperature is not detected due to a failure of a temperature sensor for the cylinder to result a driving of the screw with resin of high viscosity.
In the method of determining the load on the screw by moving the screw backward, the load on the screw by the resin remaining unmolten in grooves of the screw head, which is thick and thus requires a long time to be molten, can not detected.
FIGS. 6 and 7 schematically show the distal end of the cylinder 1. FIG. 7 is a sectional view along a line VII--VII in FIG. 6. The molten resin 7a accumulated in the distal and of the cylinder 1, the grooves of the screw head and the nozzle 2. The solidified resin in this area requires a long time for remelting and exercise a great influence on the load exerted on the screw when the screw is driven to rotate or move axially. If the screw and the screw head are completely fixed to the cylinder, the screw 3 is not moved by driving the screw backward so that an excessive load is detected. However, in the state where the resin is molted to a certain degree to have low viscosity, the screw moves backward by the backward driving to give a determination that the resin is adequately molten. When the screw is driven to move forward to inject the resin from the nozzle 2, the resistance of resin flowing through an injection hole at a distal end thereof and a connecting portion of the cylinder 1 and the nozzle 2 increases when the resin has high viscosity to exert a great load on the screw and the screw head. Thus, there is a great difference between the loads exerted on the screw when the screw is driven to move forward and backward. In the method of detecting fluidity of resin by driving the screw to move backward, the excessive load exerted on the screw w hen the screw is driven to move forward which is greater than the load exerted on the screw when the screw is driven to move forward.